Process for producing high molecular weight polyethylene

ABSTRACT

Processes for producing an (ultra) high molecular weight polyethylene (HMWPE) article include incorporating into the HMWPE resin a Hindered Amine Light Stabilizer (HALS) and cross-linking the (U)HMWPE during or after molding the (U)HMWPE resin.

This application is a continuation of commonly owned co-pending U.S.application Ser. No. 15/048,926, filed Feb. 19, 2016 (now U.S. Pat. No.______), which is a continuation of Ser. No. 14/670,742, filed Mar. 27,2015, (now U.S. Pat. No. 9,290,629), which is a continuation of U.S.application Ser. No. 12/741,431, filed Sep. 13, 2010 (now U.S. Pat. No.9,238,719) which is the national phase application under 35 USC §371 ofPCT/EP2008/065089, filed Nov. 6, 2008, which designated the U.S. andclaims priority to EP Application No. 07120102.4, filed Nov. 6, 2007 andEP Application No. 07120108.1, filed Nov. 6, 2007, the entire contentsof each of which are hereby incorporated by reference.

The present invention relates to a process for producing an (ultra) highmolecular weight polyethylene ((U)HMWPE) article. The present inventionfurther relates to an (U)HMWPE article obtainable by said process, useof an article in a medical application, and to the use of stabilizersfor the stabilization of (U)HMWPE.

Excellent properties in terms of wear, fatigue and fracture resistancehave made (U)HMWPE the material of choice in orthopedics, especially thefabrication of articular components for arthroplastry, for which a highwear resistance is required. The acetubular cup or liner in a total hipjoint replacement and the tibial insert in a total knee jointreplacement are important applications of (U)HMWPE.

HMWPE is herein defined as a substantially linear ethylene homopolymeror copolymer with a weight average molecular weight (Mw) of 3.10⁵ g/molor more, a molecular weight distribution (M_(w)/M_(n)) of between 2 and18 and an intrinsic viscosity (IV) of 1,5-8 dl/g. Preferably, the IV ofHMWPE is 3-8 dl/g and more preferably, 5-8 dl/g. The IV is definedaccording to ISO 1628-3. UHMWPE is herein defined as a substantiallylinear ethylene homopolymer or copolymer with a weight average molecularweight (Mw) of 10⁶ g/mol or more, a molecular weight distribution(M_(w)/M_(n)) of between 2 and 18 and an IV of 8 dl/g or more.Preferably, the IV of UHMWPE is between 8 and 60 dl/g.

(U)HMWPE can be obtained by any known process for the production of(U)HMWPE, as described by for example Steven M. Kurtz in “The UHMWPEHandbook”, Elsevier Academic Press, 2004, p. 14-22. (U)HMWPE isgenerally obtained as a powder which can further be processed by moldingand machining as described below.

Studies have shown that cross-linking of (U)HMWPE with gamma or electronbeam rays is highly effective against wear, which was most clearlydemonstrated for smooth counterfaces, such as those generally involvedin a prosthetic coupling.

However, despite the outstanding success of the use of (cross-linked)(U)HMWPE in total joint replacement surgery, failures arising from, forexample, aseptic loosening or mechanical failure of the component afterfew years of implantation are still quite frequent, as shown in J. H.Dumbleton et al., J. Arthroplasty 2002, 17 (5): 649-661 and T. W. Baueret al., Skeletal Radiol 1999, 28 (9): 483-497. It has been demonstratedthat most of the failures were to be ascribed to the wear of the(U)HMWPE component. Wear is a major concern, since wear leads to theformation of debris, which in turn induces an inflammatory response,causing loosening of the implant.

It has been found that the failure of the components due to is at leastpartly due to a reduction of the oxidative stability of (U)HMWPE, whichis a side-effects of irradiation used for cross-linking and/orsterilization. Irradiation was found to induce, in addition tocross-linking which has a positive effect on the mechanical properties,oxidative degradation of polyethylene which has a negative effect on themechanical properties of (U)HMWPE.

To overcome this problem, irradiated (U)HMWPE is often annealed andremelted to reduce the amount of free radicals. However, thesetreatments have a negative effect on the mechanical properties, such asthe yield and the ultimate strength.

In EP-0995450B1 vitamin E is coated on UHMWPE powder to avoid oxidationof the UHMWPE as a result of sterilization by gamma radiation. Coatingis accomplished by impregnating the UHMWPE powder with a solutioncomprising vitamin E followed by evaporation of the solvent.Subsequently the impregnated product is molded and machined into animplant which is irradiated with gamma radiation.

Disadvantages of using vitamin E as an anti-oxidant are that:

-   it results in the undesired side-effect of yellowing the (U)HMWPE.    Yellowed (U)HMWPE is perceived as an aged product in the market,-   it is consumed during cross-linking, having a negative effect on the    cross-linking efficiency and-   it has to be used in a relatively large amount to be effective    against oxidative degradation of the (U)HMWPE that occurs after    cross-linking of the (U)HMWPE.

A primary object of the invention is therefore to at least provide analternative process for the production of an (U)HMWPE article. Inparticular it is an object of the invention to provide a processresulting in a (U)HMWPE article which is less yellow, comprises,preferably, a smaller amount of stabilizer and preferably has at leastthe same stability and mechanical properties as the (U)HMWPE article ofthe prior art.

Surprisingly, it was found that the object of the invention can bereached by providing a process for producing an (ultra) high molecularweight polyethylene ((U)HMWPE) article comprising:

-   incorporating into the (U)HMWPE resin a Hindered Amine Light    Stabilizer (HALS) and-   cross-link the (U)HMWPE during or after molding the (U)HMWPE resin.

In a first embodiment of the invention the process for producing an(U)HMWPE article can comprise the following steps:

-   a) incorporating into (U)HMWPE resin a Hindered Amine Light    Stabilizer (HALS) according to one of the following general formulas    or combinations hereof:

wherein R₁ up to and including R₅ are herein independent substituents;for example containing hydrogen, ether, ester, amine, amide, alkyl,alkenyl, alkynyl, aralkyl, cycloalkyl and/or aryl groups, whichsubstituents may in turn contain functional groups, for examplealcohols, ketones, anhydrides, imines, siloxanes, ethers, carboxylgroups, aldehydes, esters, amides, imides, amines, nitriles, ethers,urethanes and any combination thereof;

-   b) molding or extruding the (U)HMWPE resin comprising the HALS,    resulting in an article;-   c) cross-linking and sterilizing the article via gamma radiation or    electron beam radiation;-   d) optionally, if step b results in a stock shape, machining the    stock shape into an article; wherein step c and step d can be    performed in either order.

In a second embodiment of the invention the process for producing an(U)HMWPE article can comprise the following steps:

-   a) incorporating into (U)HMWPE resin a Hindered Amine Light    Stabilizer (HALS) according to one of the following general formulas    or combinations hereof:

wherein R₁ up to and including R₅ are herein independent substituents;for example containing hydrogen, ether, ester, amine, amide, alkyl,alkenyl, alkynyl, aralkyl, cycloalkyl and/or aryl groups, whichsubstituents may in turn contain functional groups, for examplealcohols, ketones, anhydrides, imines, siloxanes, ethers, carboxylgroups, aldehydes, esters, amides, imides, amines, nitriles, ethers,urethanes and any combination thereof;

-   b) adding a an initiator, for example a peroxide, and optionally a    coagent;-   c) molding or extruding the (U)HMWPE resin comprising the HALS and    the initiator, resulting in a stock shape or an article;-   d) optionally, when further cross-linking or sterilizing via gamma    radiation or electron beam radiation is applied, cross-linking    and/or sterilizing the stock shape or the article;-   e) optionally, if step c results in a stock shape, machining the    stock shape into an article; wherein step d and step e can be    performed in either order.

Surprisingly, a much lower amount of the HALS can be used compared withthe amount of vitamin E for stabilizing the (U)HMWPE article.

The HALS is preferably used in an amount of between 0.001 and 5% byweight, more preferably between 0.01 and 2% by weight, most preferablybetween 0.02 and 1% by weight, based on the total weight of the(U)HMWPE.

Preferably, the HALS chosen is a compound derived from a substitutedpiperidine compound, in particular any compound which is derived from analkyl-substituted piperidyl, piperidinyl or piperazinone compound or asubstituted alkoxypiperidinyl compound.

Examples of such compounds are:

-   2,2,6,6-tetramethyl-4-piperidone; 2,2,6,6-tetramethyl-4-piperidinol;    bis-(1,2,2,6,6-pentamethylpiperidyl)-(3′,5′-di-tert-butyl-4′-hydroxybenzyl)    butylmalonate; di-(2,2,6,6-tetramethyl-4-piperidyl) sebacate    (Tinuvin® 770); oligomer of    N-(2-hydroxyethyl)-2,2,6,6-tetramethyl-4-piperidinol and succinic    acid (Tinuvin® 622); bis-(2,2,6,6-tetramethyl-4-piperidinyl)    succinate; bis-(1-octyloxy-2,2,6,6-tetramethyl-4-piperidinyl)    sebacate (Tinuvin® 123); bis-(1,2,2,6,6-pentamethyl-4-piperidinyl)    sebacate (Tinuvin® 765); N,N′-bis-(2,2,6,6-tetramethyl-4-piperidyl)    hexane-1,6-diamine (Chimassorb® T5);    N-butyl-2,2,6,6-tetramethyl-4-piperidinamine;    2,2′-[(2,2,6,6-tetramethylpiperidinyl)imino]-bis-[ethanol];    poly((6-morpholine-S-triazine-2,4-diyl)(2,2,6,6-tetramethyl-4-piperidinyl)-iminohexamethylene-(2,2,6,6-tetramethyl-4-piperidinyl)-imino)    (Cyasorb® UV 3346);    5-(2,2,6,6-tetramethyl-4-piperidinyl)-2-cyclo-undecyloxazole)    (Hostavin® N20);    8-acetyl-3-dodecyl-7,7,9,9-tetramethyl-1,3,8-triaza-spiro(4,5)decane-2,4-dione;    polymethylpropyl-3-oxy[4(2,2,6,6-tetramethyl)piperidinyl)siloxane    (Uvasil® 299); copolymer of    α-methylstyrene-N-(2,2,6,6-tetramethyl-4-piperidinyl)maleimide and    N-stearylmaleimide; 1,2,3,4-butanetetracarboxylic acid, polymer with    beta, beta,    beta′,beta′-tetramethyl-2,4,8,10-tetraoxaspiro[5.5]undecane-3,9-diethanol,    1,2,2,6,6-pentamethyl-4-piperidinyl ester (Mark® LA63);    2,4,8,10-tetraoxaspiro[5.5]undecane-3,9-diethanol,beta,beta,beta′,beta′-tetramethyl-,    polymer with 1,2,3,4-butanetetracarboxylic acid,    2,2,6,6-tetramethyl-4-piperidinyl ester (Mark® LA68); D-glucitol,    1,3:2,4-bis-O-(2,2,6,6-tetramethyl-4-piperidinylidene)-(HALS 7);    oligomer of    7-oxa-3,20-diazadispiro[5.1.11.2]heneicosan-21-one,2,2,4,4-tetramethyl-20-(oxiranylmethyl)    (Hostavin® N30); propanedioic acid,    [(4-methoxyphenyl)methylene]-,bis(1,2,2,6,6-pentamethyl-4-piperidinyl)    ester (Sanduvor® PR 31); formamide,    N,N′-1,6-hexanediylbis[N-(2,2,6,6-tetramethyl-4-piperidinyl (Uvinul®    4050H). 1,3,5-triazine-2,4,6-triamine, N,N′″-[1,2-ethanediylbis    [[[4,6-bis[butyl(1,2,2,6,6-pentamethyl-4-iperidinyl)amino]-1,3,5-triazine-2-yl]imino]-3,1-propanediyl]]-bis[N′,N″-dibutyl-N′,N″-bis(1,2,2,6,6-pentamethyl-4-piperidinyl)    (Chimassorb® 119); 1,5-dioxaspiro (5,5) undecane 3,3-dicarboxylic    acid, bis (2,2,6,6-tetramethyl-4-peridinyl) ester (Cyasorb® UV-500);    1,5-dioxaspiro (5,5) undecane 3,3-dicarboxylic acid, bis    (1,2,2,6,6-pentamethyl-4-peridinyl) ester (Cyasorb® UV-516);    N-2,2,6,6-tetramethyl-4-piperidinyl-N-amino-oxamide;    4-acryloyloxy-1,2,2,6,6-pentamethyl-4-piperidine; HALS PB-41    (Clariant Huningue S.A.);    1,3-benzendicarboxamide,N,N′-bis(2,2,6,6-tetramethyl-4-piperidinyl)    (Nylostab® S-EED (Clariant Huningue S.A.));    3-dodecyl-1-(2,2,6,6-tetramethyl-4-piperidyl)-pyrrolidin-2,5-dione;    1,3-Propanediamine, N,N-1,2-ethanediylbis-,polymer with    2,4,6-trichloro-1,3,5-triazine, reaction products with    N-butyl-2,2,6,6-tetramethyl-4-piperidinamine (Uvasorb® HA88);    1,1′-(1,2-ethane-di-yl)-bis-(3,3′,5,5′-tetra-methyl-piperazinone)    (Good-rite® 3034); 1,1′,1″-(1,3,5-triazine-2,4,6-triyltris    ((cyclohexylimino)-2,1-ethanediyl)tris(3,3,5,5-tetramethylpiperazinone);    (Good-rite® 3150);    1,1′,1″-(1,3,5-triazine-2,4,6-triyltris((cyclohexylimino)-2,1-ethanediyl)tris(3,3,4,5,5-tetramethylpiperazinone)    (Good-rite@ 3159); 1,2,3,4-Butanetetracarboxylic acid,    tetrakis(2,2,6,6-tetramethyl-4-piperidinyl) ester (ADK STAB@ LA-57)    1,2,3,4-Butane-tetra-carboxyllc acid,    1,2,3-tris-(1,2,2,6,6-penta-methyl-4-piperidyl)-4-tridecylester (ADK    STAB@ LA-62).

Mixture of esters of 2,2,6,6-tetra-methyl-4-pipiridinol and severalfatty acid (CYASORB® UV3853); Propanedioic acid,[(4-methoxyphenyl)methylene]-,bis(2,2,6,6-tetramethyl-4-piperidinyl)ester (HOSTAVIN® PR-31);3-Dodecyl-1-(2,2,6,6-tetramethyl-4-piperidyl)-pyrrolidin-2,5-dione(CYASORB® UV3581);3-Dodecyl-1-(1,2,2,6,6-pentamethyl-4-piperidyl)-pyrrolidin-2,5-dione(CYASORB® UV3641); 1,2,3,4-Butanetetracarboxylic acid,tetrakis(1,2,2,6,6-pentamethyl-4-piperidinyl)ester (ADK STAB@ LA-52);1,2,3,4-Butane-tetra-carboxyllc acid,1,2,3-tris-(2,2,6,6-tetra-methyl-4-piperidyl)-4-tridecylester (ADK STAB@LA-67); Mixture of: 2,2,4,4tetramethyl-21-oxo-7-oxa-3.20-diazadispiro[5.1.11.2]-heneicosane-20-propionicacid dodecylester and 2,2,4,4tetramethyl-21-oxo-7-oxa-3.20-diazadispiro[5.1.11.2]-heneicosane-20-propionicacidtetradecylester (Hostavin® N24);Poly[[6-[(1,1,3,3-tetramethylbutyl)amino]-s-triazine-2,4-diyl][2,2,6,6-tetramethyl-4-piperidinyl)-imino]hexamethylene[(2,2,6,6-tetramethyl-4-piperidinyl) imino]]} (Chimassorb® 944);1,3,5-Triazine-2,4,6-triamine, N,N′″-[1,2-ethanediylbis[[[4,6-bis[butyl(1,2,2,6,6-pentamethyl-4-piperidinyl)amino]-1,3,5-triazine-2-yl]imino]-3,1-propanediyl]]-bis[N′,N″-dibutyl-N′,N″-bis(1,2,2,6,6-pentamethyl-4-piperidinyl)(Chimassorb® 119);Poly[(6-morpholino-s-triazine-2,4-diyl)[1,2,2,6,6-penta-methyl-4-piperidyl)imino]-hexamethylene[(1,2,2,6,6penta-methyl-4-piperidyl)imino]]1,6-Hexanediamine,N,N′-bis(1,2,2,6,6-pentamethyl-4-pipiridinyl)-, Polymers withmorpholine-2,4,6-trichloro-1,3,5-triazine (CYASORB® UV3529);Poly-methoxypopyl-3-oxy[4(1,2,2,6,6-pentamethyl)-piperidinyl]-siloxane(Uvasil®816); 1,6-Hexanediamine, N,N′-bis(2,2,6,6-tetramethyl-4piperidinyl)-polymer with2,4,6-trichloro-1,3,5-triazine, reaction products withN-butyl-1-butanamine and N-butyl-2,2,6,6-tetramethyl-4-piperidinamine(Chimassorb® 2020); Reaction products of N, N′-ethane-1,2-diylbis(1,3-propanediamine), cyclohexane, peroxidized4-butylamino-2,2,6,6-tetramethylpiperidine and2,4,6-trichloro-1,3,5-triazine (Flamestab NOR® 116); 1,6-hexanediamine,N,N′-bis(2,2,6,6-tetramethyl-4-piperidinyl)-, polymer with2,4,6-trichloro-1,3,5-triazine, reaction products with3-bromo-1-propene, n-butyl-1-butanamine andN-butyl-2,2,6,6-tetramethyl-4-piperidinamine, oxidised, hydrogenated(Tinuvin NOR® 371).

Preferably, the HALS has a molecular mass of 1000 g/mol or more, morepreferably 1500 g/mol or more, or contains a group via which it can begrafted to the (U)HMWPE. Both measures help to avoid leaching of thestabilizer from the product during use.

The HALS can be incorporated in (U)HMWPE by any known method. The HALS,either as liquid or powder, can be mixed with the (U)HMWPE resin or withthe (U)HMWPE melt. Alternatively, (U)HMWPE resin may be impregnated witha solution of the HALS, or a solution of the HALS may be sprayed on the(U)HMWPE resin. Also (U)HMWPE particles may be mixed with the HALS insupercritical CO₂. Dependent on the HALS type, examples of suitablesolvents include methanol, ethanol, butanol, isopropyl alcohol,ethylglycol, ethyl acetate, tetrahydrofuran, acetone,methylisobutylketone, chloroform, methylene chloride, hexane, toluene,and xylene. Further the HALS stablilizer can be incorporated in the(U)HMWPE during polymerization. This has the advantage that a veryhomogeneous distribution of the HALS in the (U)HMWPE can be obtained.

According to one embodiment of the invention the (U)HMWPE iscross-linked by irradiation by applying for example gamma irradiation orelectron beam irradiation, as described in the above-referenced “TheUHMWPE Handbook” on p. 37-47. The irradiation dose used to obtain ahighly cross-linked (U)HMWPE article is chosen between 30 and 250 kGray(kGy), preferably between 30 and 170 kGy and more preferably between 40and 130 kGy. To obtain a lower cross-linked (U)HMWPE article or whenirradiation is used in combination with chemical cross-linking by theuse of an initiator a lower irradiation dose can be used, from forinstance 25 to 50 kGy.

For sterilization of the (U)HMWPE article according to the invention anirradiation dose between 10 and 40 kGy, preferably between 20 and 35 kGycan be used.

According to a second embodiment of the invention the (U)HMWPE iscross-linked by adding an initiator, for example a peroxide, andoptionally a coagent to the (U)HMWPE.

Examples of suitable peroxides include tert-butyl cumyl peroxide,tert-butyl peroxybenzoate, di-tert-butyl peroxide,3,3,5,7,7-pentamethyl-1,2,4-trioxepane,1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane,butyl 4,4-di(tert-butylperoxy)valerate,2,5-dimethyl-2,5-di(tert-butylperoxy)hexyne-3,2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, di(4-methylbenzoyl)peroxide, dibenzoyl peroxide, di(2,4-dichlorobenzoyl) peroxide, dicumylperoxide, 3,3,5,7,7-pentamethyl-1,2,4-trioxepane,1-(2-tert.-butylperoxyisopropyl)-3-isopropenyl benzene,2,4-diallyloxy-6-tert-butylperoxy-1,3,5-triazine,di(tert-butylperoxyisopropyl)benzene, diisopropylbenzenemonohydroperoxide, cumyl hydroperoxide, and tert-butyl hydroperoxide.

Optionally a coagent, a compound with 2 or more unsaturations, is usedto enhance the peroxide cross-linking efficiency. Examples of suitablecoagents include divinylbenzene, diallylphthalate, triallylcyanurate,triallylisocyanuarate, triallyltrimellitate, meta-phenylenebismaelimide, ethyleneglycol dimethacrylate, ethyleneglycol diacrylate,trimethylopropane timethacrylate, trimethylopropane, timethacrylate,pentaerytritol tetramethacrylate, zinc diacrylate, zinc dimethacrylate,and polybutadiene.

The initiator is generally used in an amount of between 0.001 and 2.5%by weight, preferably between 0.01 and 1% by weight, and the co-agent inan amount of between 0.001 and 2.5% by weight, preferably between 0.1and 1% by weight, both based on the total weight of the (U)HMWPE.

The HALS, optionally the initiator, for example peroxide, and optionallythe coagent, can be incorporated in the (U)HMWPE in step a andoptionally b, and then be consolidated by compression molding or hotisostatic pressing. The HALS, optionally the initiator and optionallythe coagent can be incorporated in (U)HMWPE by any known method. TheHALS, optionally the initiator and optionally the coagent, either asliquid or powder, can be mixed with the (U)HMWPE resin or melt.Alternatively, (U)HMWPE resin may be impregnated with a solution of theHALS and optionally the initiator in a solvent, spraying a solution ofthe HALS and optionally the initiator on (U)HMWPE resin, and mixing(U)HMWPE particles with the HALS and optionally the initiator insupercritical CO₂. In all methods the impregnation of the HALS andoptionally the initiator may take place simultaneously or separately,i.e. one after the other.

Cross-linking takes place in the melt during molding and/or viaradiation. Several processing methods can be used for molding and, ifnecessary, machining (optional step) of (U)HMWPE resins into bulkproducts, as described in for example Steven M. Kurtz in “The UHMWPEHandbook”, Elsevier Academic Press, 2004, p. 22-31 (hereinafter “TheUHMWPE Handbook”), which is incorporated herein as a reference. A shortdescription of the main methods is described below.

The first method is compression molding, wherein a mold filled with(U)HMWPE resin is subjected to a combination of high temperature andhigh pressure for a certain amount of time. Subsequently the system iscooled at a slow and uniform rate in order to minimize shrinkage anddeformation of the product. The product is than machined into smallerblocks or cylindrical bars from which the final components, for examplearticular components, can be machined.

The second method, ram extrusion, produces cylindrical bar stock shapesranging in diameter from 25 mm to 150 mm. In this process, the (U)HMWPEresin charge is fed into a channel and then heat is applied. A ram thencompresses and extrudes the plasticized powder charge into the heatedcylindrical barrel, where it is consolidated into a cylindrical barstock. As the ram moves back and forward, the powder stock in thechamber is refilled. The final components, for example articularcomponents, can be machined from the bar stock.

In the third method, direct compression molding, the (U)HMWPE resincharge is consolidated into a final or semifinal bulk product using apre-shaped mold. Machining is not always necessary when this method isapplied. Although this process is slow and costly, orthopedic articularcomponents made using this method have very smooth surface finishes andexcellent dimensional consistency.

A HMWPE resin can also be melt processed by injection molding orextrusion into a sheet or a bar. In this way the end product can bedirectly obtained, but machining the product to obtain the end productis also possible.

In addition to the above methods Hot Isostatic Pressing (HIPing) can beapplied, as described in The UHMWPE Handbook on p. 27.

Machining of (U)HMWPE consists of milling and turning operations forboth first rough and finishing steps. More details about machining areprovided in the above-referenced “The UHMWPE Handbook” on p. 31-32.

The methods that are used for cross-linking (U)HMWPE by radiation, andwhich can also be applied to (U)HMWPE stabilized with the HALS and, oroptionally, chemically cross-linked according to the invention, aregamma irradiation and electron beam irradiation, as described by G.Lewis in Biomaterials 2001, 22: 371-401. With gamma irradiation, thedosage used ranges from 20-1000 kGy. Either a stock shape or an article,or both, can be subjected to irradiation cross-linking applying electronbeam or gamma radiation.

Optionally, the stock shape or article is annealed after cross-linkingat a temperature below the melting temperature of (U)HMWPE, for examplebetween 60 and 140° C.

In addition to ethylene, (U)HMWPE may comprise one or more comonomers,for example propylene, butane, pentene, hexane, 4-methylpentene, octane,octadiene and vinylnorbornene, and the like, to achieve improvedprocessing characteristics or alter the physical and mechanicalproperties of the polymer. Furthermore the polymer can containnon-polymer materials such as reinforcing agents, fillers, flameretardants, pigments, and other auxiliary additives like plasticizers,processing aids, such as mould release agents, further stabilizers suchas antioxidants and UV stabilizers, crystallization accelerating agentsor nucleating agents, impact modifiers and compatibilizers. Inparticular, an inorganic stearate such as calcium or zinc stearate maybe added to the (U)HMWPE-P resin as a flow agent or to minimize theeffect of any catalyst residues, which have a potential for corrodingthe conversion machines. Moreover, calcium stearate may act as alubricant when a part is to be fabricated using ram-extrusion of thepolymer powders, and may help the product to maintain its white color.

The (U)HMWPE article comprising a HALS has a cross-link density of 0.09mol/dm³or more.

The (U)HMWPE article according to the invention can be applied inmedical applications, preferably in implants which have a thickness ofat least 2 mm, more preferably at least 4 mm. For example the implantscan be used in orthopedics as bearing material in artificial joints.(U)HMWPE can be used in for example hip arthroplasty, knee replacements,shoulder replacements and spinal applications such as total discreplacement. These applications are described in detail in theabove-referenced “The UHMWPE Handbook” in Chapters 4-6 (hip), 7-8(knee), 9 (shoulder) and 10 (spinal applications).

The present invention will now be described in detail with reference tothe following examples that by no means limit the scope of theinvention.

EXAMPLES Materials UHMWPE

The used UHMWPE had an Intrinsic Viscosity, measured according to ISO1628-3, of 27 dl/g, which corresponds with a molecular weight of 7.3million g/mol, as calculated using Margolies equationMw=53700×[I.V]^(1.49)

The average particle size of the used UHMWPE resin according to ISO13320 was 157 micron.

Stabilizers

Vitamin E; (Alpha tocopherol from DSM Nutrional Products)

Poly{[[6-[(1,1,3,3-tetramethylbutyl)amino]-s-triazine-2,4-diyl][(2,2,6,6-tetramethyl-4-piperidinyl)imino]-hexamethylene[(2,2,6,6-tetramethyl-4-piperidinyl) imino]]}; (Chimassorb® 944 fromCiba Specialty Chemicals)

1,3,5-Triazine-2,4,6-triamine, N,N′″-[1,2-ethanediylbis[[[4,6-bis[butyl(1,2,2,6,6-pentamethyl-4-piperidinyl)amino]-1,3,5-triazine-2-yl]imino]-3,1-propanediyl]]-bis[N′,N″-dibutyl-N′,N″-bis(1,2,2,6,6-pentamethyl-4-piperidinyl);(Chimassorb® 119 from Ciba Specialty Chemicals)

1,6-hexanediamine, N,N′-bis(2,2,6,6-tetramethyl-4-piperidinyl)-, polymerwith 2,4,6-trichloro-1,3,5-triazine, reaction products with3-bromo-1-propene, n-butyl-1-butanamine andN-butyl-2,2,6,6-tetramethyl-4-piperidinamine, oxidised, hydrogenated;(Tinuvin® NOR 371 from Ciba Specialty Chemicals)

Preparation of Solvent Blended Compounds

The stabilizers were added to the UHMWPE by solvent blending. Thestabilizers were first added to the polymer as a solution in Chloroform(about 100 ml/100 gr polymer); in a second step the chloroform wasevaporated.

Irradiation of the Samples

Irradiation was performed by gamma irradiation (dose 25, 75 and 150kGray (kGy)) on stock samples under nitrogen that were vacuum sealedinto paper bags with an aluminum coating on the inside. To prepare thetest samples for the Swell test a stock sample was prepared that wasirradiated and was later machined into the smaller test samples.

Preparation of the Samples

Powder was compression molded into samples according to ISO 11542-2.

Needed sample dimensions for analyses were machined from the moldedstock samples.

Ageing

The samples for the tensile test and for color determination were agedduring two weeks in an air venting oven (Binder FDL115) at 110° C.

Cross-Link Density Measurement

The cross-link density was determined according to ASTM F2214-02, usingsamples with the dimensions 5 mm*5 mm*5 mm that were machined out ofstock samples that were irradiated. These samples were subjected toswelling in o-xylene.

Colour Determination

Colour determination was done according to ISO 7724-1-2-3 (CIELAB, D65,10°, d8). The determination was done in reflection using a blackbackground with a Minolta spectrophotometer. As samples 1 mm thickplaques were used that were machined out of the stock samples afterirradiation and ageing.

Tensile Tests

The tensile tests (elongation at break and ultimate tensile strength)were performed according to ISO 527.

Tensile bars (Type ISO 527-5B) were punched from 1 mm thick samples,that were machined out of stock samples after irradiation and ageing.

Oxidation Index Determination

The oxidation indices were determined from the Infrared Spectra measuredin transmission on coupes of about 100 μm, which were cut from cubes of5*5*5 mm. The spectra were recorded on a Perkin Elmer Auto Image using20 scans and a resolution of 4 cm⁻¹. The spectra were normalized as inASTM F2102-06 to 1370 cm⁻¹ (1330-1370, base 1400 cm⁻¹). The oxidationindex was defined as the peak height at 1717 cm⁻¹ using a baseline drawnfrom 1680-1765 cm^(−1.)

Results

TABLE 1 Color determination Radiation dose Example Stabilizer 0 kGy 25kGy 75 kGy 150 kGy A — 0 0.55 1.35 2.88 B 0.15 wt % 7.71 12.16 12.0713.07 Vitamin E 1 0.05 wt % −0.14 0.47 1.39 3.00 Chimassorb 944 2 0.15wt % −0.13 0.30 1.31 2.92 Chimassorb 944 3 0.05 wt % −0.21 0.41 1.312.84 Chimassorb 119 4 0.15 wt % −0.23 0.2 1.23 2.91 Chimassorb 119 50.05 wt % 0.36 0.46 1.48 2.93 Tinuvin NOR 371 6 0.15 wt % 0.94 1.44 2.293.41 Tinuvin NOR 371

In table 1 the difference between the color of the b*-value of thedifferent samples and the color of the reference sample (not stabilized,not irradiated sample) is given.

From these results it was clear that the Vitamin E containing sampleswere more yellow than the HALS containing samples.

TABLE 2 Cross-link density Cross-link density (Mol/dm³) for samples thatwere irradiated with different doses. Radiation dose Example Stabilizer25 kGy 75 kGy 150 kGy C — 0.148 0.223 0.234 D 0.15 wt % Vitamin E 0.0870.160 0.215 7 0.05 wt % Chimassorb 944 0.145 0.258 0.335 8 0.15 wt %Chimassorb 944 0.157 0.233 0.381 9 0.05 wt % Chimassorb 119 0.114 0.2130.196 10 0.15 wt % Chimassorb 119 0.131 0.187 0.248 11 0.05 wt % TinuvinNOR 371 0.151 0.209 0.279 12 0.15 wt % Tinuvin NOR 371 0.120 0.162 0.323

From the results in Table 2 it is clear that for the HALS stabilizedsamples a lower radiation dose is needed to get a cross-link densitythat is comparable with the Vitamin E containing sample.

TABLE 3 Tensile strength Tensile strength (N/mm²) of samples that wereirradiated with different doses, after ageing for two weeks at 110° C.Radiation dose 0 25 75 150 Example Stabilizer kGy kGy kGy kGy E — 11.610.8 13.6 17.4 F 0.15 wt % Vitamin E 55 51 48.7 41.6 13 0.05 wt %Chimassorb 944 56.9 50.3 45.9 42.4 14 0.05 wt % Chimassorb 119 55.4 49.645.3 41.8 15 0.05 wt % Tinuvin NOR 371 56.2 48.5 45.1 40.6

From these results it was clear that after ageing samples comprising0.05 wt % HALS had a tensile strength that was comparable with a tensilestrength for a sample comprising 0.15 wt % Vitamin E.

TABLE 4 Oxidation index Oxidation index of samples that were irradiatedwith different doses after ageing for two weeks at 110° C. Radiationdose 0 25 75 150 Example Stabilizer kGy kGy kGy kGy G — 9.74 8.96 10.58.8 H 0.15 wt % Vitamin E 0.012 0.042 0.095 0.215 16 0.05 wt %Chimassorb 944 0.112 0.019 0.184 0.239 17 0.05 wt % Chimassorb 119 0.0240.139 0.276 0.210 18 0.05 wt % Tinuvin NOR 371 0.108 0.212 0.224 0.293

From these results it was clear that 0.05 wt % HALS could prevent anincrease of the oxidation index. The amount needed from the HALS waslower than the 0.15 wt % Vitamin E that was needed to obtain the sameresult.

TABLE 5 Change in cross-link density Change in cross-link density (inmol/dm³) of samples that were irradiated with different doses afterageing for two weeks at 110° C. Radiation dose Example Stabilizer 25 kGy75 kGy I — −0.1 −0.2 J 0.15 wt % Vitamin E 0.0 0.0 19 0.05 wt %Chimassorb 944 0.0 0.0 20 0.05 wt % Chimassorb 119 0.0 0.0 21 0.05 wt %Tinuvin NOR 371 0.0 0.0

From these results it was clear that the HALS, as well as Vitamin E,were effective in preventing a decrease in cross-link density due toageing.

1. A process for making a cross-linked article from (ultra) highmolecular weight polyethylene ((U)HMWPE) comprising steps of: forming acomposition by incorporating at least one Hindered Amine LightStabilizer (HALS) compound in a (U)HMWPE resin, which (U)HMWPE is asubstantially linear ethylene homopolymer or copolymer with a molecularweight distribution (M_(w)/M_(n)) of between 2 and 18; adding aninitiator, for example a peroxide, to the composition; and molding thecomposition to form an article having a cross-link density of 0.09mol/dm³ or more.
 2. The process according to claim 1 comprising thesteps of: a) forming a composition by incorporating in the (U)HMWPE aHALS compound according to one of the following general formulas orcombinations hereof:

wherein R₁ up to and including R₅ are herein independent substituents;for example containing hydrogen, ether, ester, amine, amide, alkyl,alkenyl, alkynyl, aralkyl, cycloalkyl and/or aryl groups, whichsubstituents may in turn contain functional groups, for examplealcohols, ketones, anhydrides, imines, siloxanes, ethers, carboxylgroups, aldehydes, esters, amides, imides, amines, nitriles, ethers,urethanes and any combination thereof; b) adding an initiator andoptionally a coagent to the composition; c) molding or extruding thecomposition to form a cross-linked article or stock shape; d) optionallyfurther cross-linking and/or sterilizing the article or stock shape viagamma radiation or electron beam radiation; e) optionally, if step c)results in a stock shape, machining the stock shape into an article,wherein step d) and step e) can be performed in either order.
 3. Theprocess according to claim 1, wherein the (U)HMWPE has an intrinsicviscosity of 8 dl/g or more.
 4. The process according to claim 1,wherein the (U)HMWPE comprises one or more comonomers.
 5. The processaccording to claim 1, wherein the HALS is present in an amount ofbetween 0.01 and 2% by weight, based on the total weight of the(U)HMWPE.
 6. The process according to claim 1, wherein the HALS has amolecular weight of 1000 g/mol or more.
 7. The process according toclaim 1, wherein the HALS compound is selected from the group consistingof N,N′,″-[1,2-ethanediylbis[[[4,6-bis[butyl(1,2,2,6,6-pentamethyl-4-iperidinyl)amino]-1,3,5-triazine-2-yl]imino]-3,1-propanediyl]]-bis[N′,N″-dibutyl-N′,N″-bis(1,2,2,6,6-pentamethyl-4-piperidinyl)orpoly{[[6-[(1,1,3,3-tetramethylbutyl)amino]-s-triazine-2,4-diyl][(2,2,6,6-tetramethyl-4-piperidinyl)imino]-hexamethylene[(2,2,6,6-tetramethyl-4-piperidinyl) imino]]}.
 8. The process accordingto claims 1, wherein the HALS is incorporated in the (U)HMWPE by mixingthe HALS with (U)HMWPE resin or (U)HMWPE melt, by impregnating (U)HMWPEresin with a solution comprising the HALS, or by spraying a solutioncomprising the HALS on (U)HMWPE resin.
 9. The process according to claim1, wherein the initiator is a peroxide chosen from the group consistingof tert-butyl cumyl peroxide, tert-butyl peroxybenzoate, di-tert-butylperoxide,3,3,5,7,7-pentamethyl-1,2,4-trioxepane,1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane,butyl 4,4-di(tert-butylperoxy)valerate,2,5-dimethyl-2,5-di(tert-butylperoxy)hexyne-3,2,5-dimethyl-2,5-di(tert-butylperoxy)hexane,di(4-methylbenzoyl) peroxide, dibenzoyl peroxide,di(2,4-dichlorobenzoyl) peroxide, dicumyl peroxide,3,3,5,7,7-pentamethyl-1,2,4-trioxepane,1-(2-tert.-butylperoxyisopropyl)-3-isopropenyl benzene,2,4-diallyloxy-6-tert-butylperoxy-1,3,5-triazine,di(tert-butylperoxyisopropyl)benzene, diisopropylbenzenemonohydroperoxide, cumyl hydroperoxide, and tert-butyl hydroperoxide.10. The process according to claim 1, wherein the initiator is presentin an amount of between 0.001 and 2.5% by weight based on the totalweight of the (U)HMWPE.
 11. The process according to claim 1, whereinthe initiator is present in an amount of between 0.01 and 1% by weightbased on the total weight of the (U)HMWPE.
 12. The process according toclaim 2, wherein the coagent is chosen from the group consisting ofdivinylbenzene, diallylphthalate, triallylcyanurate,triallylisocyanuarate, triallyltrimellitate, meta-phenylenebismaelimide, ethyleneglycol dimethacrylate, ethyleneglycol diacrylate,trimethylopropane timethacrylate, trimethylopropane, timethacrylate,pentaerytritol tetramethacrylate, zinc diacrylate, zinc dimethacrylate,and polybutadiene.
 13. The process according to claim 2, wherein theco-agent is present in an amount of between 0.001 and 2.5% by weightbased on the total weight of the (U)HMWPE.
 14. The process according toclaim 2, wherein the co-agent is present in an amount of between 0.1 and1% by weight based on the total weight of the (U)HMWPE.
 15. The processaccording to claim 1, wherein the step of molding or extruding comprisescompression molding, ram-extrusion, direct compression molding, or hotisostatic pressing.
 16. The process according to claim 2, wherein thestep of further cross-linking and/or sterilizing is performed with anirradiation dose of from 10 to 40 kGy.
 17. The process according toclaim 2, further comprising annealing the cross-linked article or stockshape at a temperature below the melting temperature of (U)HMWPE. 18.The process according to claim 2, further comprising annealing thecross-linked article or stock shape at a temperature of between 60 and140° C.
 19. A crosslinked (U)HMWPE article obtained by the processaccording to claim 1, wherein the article is applied in medicalimplants.
 20. The article according to claim 19, having a thickness ofat least 2 mm.